The present invention relates to a system for treating substrates by plasma and, more particularly, to a system for generating uniform plasma for processing a substrate having two major surfaces.
In the manufacture of many electronic components, such as integrated circuits, there is a need to deposit metallic films on substrates. Materials such as copper may be deposited on ceramic or glass substrates and then etched or otherwise fabricated into the electrical circuits and/or components.
In the field of plasma deposition, an atom may be displaced from the surface of a target connected to a cathode by a process called sputtering or sputter deposition. In this process, the target is most often constructed of electrically conductive material such as copper or the like. The cathode to which the target is attached is subjected to a relatively high voltage, either DC or radio frequency, in an inert atmosphere such as argon. The inert gas is ionized, forming an excited gaseous state (plasma) from which positive ions escape to bombard the exposed surface of the target. By momentum transfer, the atoms or clusters of atoms of the target material are thereby dislodged. It is this dislodging of the target atoms that is known as sputtering. By repeating this process, a number of these primarily neutral atoms move through the space in front of the target, in a relatively high vacuum. Eventually these atoms strike and condense on the surface of a receiver, known as a sample or substrate, which is generally in close proximity to the target. A coating of atomic or molecular layers of target material can thus be built on the substrate. The coating, which is generally less than 1 .mu.m, is called a thin film. It is generally sufficient for the metallization of integrated circuits.
Through holes or viaduct holes (commonly called vias) are paths for electrical interconnections between a first-level conductive pattern and a second-or higher-level conductive pattern. In order to electrically connect circuits on different substrate levels to each other, precious metal (e.g., palladium) seeding and electroless metal deposition have been used to coat the walls of the vias, often followed by electroplating. Most recently, however, plasma technology has been applied to this problem.
In the field of plasma processing of substrates for use as printed circuit boards and cards, nonuniformity of a plasma field can result in nonuniform etching, nonuniform deposition and/or nonuniform cleaning of via holes and through holes, the latter being referred to as desmearing. In the etching process, for example, a more intense plasma field in the center of a board results in a higher etching rate for that portion of the board, whereas a relatively sparse plasma density at the edges of the board results in a proportionally and predictably low etch rate for those sections. It has been found that nonuniformity of an electric field in proximity to an electrically floating printed circuit board results in nonuniform plasma treatment thereof.
Normally, oppositely charged electrodes are provided in a vacuum chamber to initiate a plasma reaction. Such apparatus is shown, for example, in copending patent application, Ser. No. 587,098 filed Mar. 7, 1984 for "Shield for Improved Magnetion Sputter Deposition into Surface Recesses" and assigned to the present assignee.
For desmearing via holes and through holes, one technique is disclosed in U.S. Pat. No. 4,230,553, issued to Bartlett, et al. This technique uses plasma etching wherein the conductive surface layers of drilled boards are themselves the electrodes that help generate plasma. The plasma forms directly within the holes to remove the smear. A radio frequency (r.f.) generator is electrically connected to one surface of each of the boards being processed. The other surface of each of the boards is grounded. The plasma generated in this system is thus present only in the through holes. One r.f. source supplies power to the system.
It should be noted, however, that, while in a uniform electric field relatively low radio frequencies can drive ions through the substrate's through holes, this practice used in a nonuniform electric field has the opposite effect: nonuniformity of the etching process is exacerbated.
U.S. Pat. No. 3,757,733, issued to Reinberg, discloses a cylindrical reactor for use in plasma deposition. Means are provided for establishing a radial flow of reactant gases from the outer region of a support that holds substrates to an inner region of the support. Means are also provided to exhaust gases from the inner region of the support. Such an apparatus, as disclosed, can be used only in batch operations to treat only one major surface of a substrate at a time, as opposed to continuous or semi-continuous operations. Moreover, since much of the reactant gases is consumed, the uniformity of plasma deposition is ensured only for relatively small substrates or wafers. Finally, the apparatus is intended to be used for plasma deposition only, not etching.
U.S. Pat. No. 4,264,393, issued to Gorin, et al, discloses a reactor apparatus for plasma etching or deposition. The apparatus has two parallel plates within the reactor, one of which functions as a manifold for reactant gases which are dispensed through a regular array of orifices into the r.f. field between the plates. The parallel plate structure can be stacked in an array of alternating grounded and r.f. plates to increase the apparatus capacity. The reactant gases are introduced only from one direction (i.e., the plates have an array of orifices only on one side), resulting in one-sided etching of substrates or via holes therein.
The aforementioned references are not appropriate for etching large substrates on both major surfaces or for generating uniform plasma fields over a large area.
It would be advantageous to provide a plasma reactor system for generating uniform plasma fields.
It would further be advantageous to provide a plasma system for uniformly processing the major surfaces as well as the via holes and through holes of large substrates.
It would also be advantageous to process substrates in uniform plasma and electrical fields continuously or semi-continuously, replenishing reactant gases as they are consumed.
It would further be advantageous to use a plasma reactor in continuous fashion for etching and cleaning substrates.
It would be advantageous to provide for bi-directional plasma gas flow against, around and through the substrates for more uniform and efficient plasma processing.